What's an HTPC?
HTPC is an acronym for Home Theater PC.
Basically, it's a PC in a case that can fit onto a shelf or rack, like a DVD player. It hooks up to your TV, and can function like a normal PC, in addition to giving you TiVo-like functionality (without paying the subscription!) Set up a home network and you can stream DVDs from a central server and watch them on your TV, without having to get up and load DVDs.
They're extremely useful pieces of computing, and will eventually replace the desktop, IMO.